This invention relates to copper alloy foils with excellent strength and heat resistance, and excellent productivity. The copper alloy foils according to the invention are suited for such applications as flexible printed wiring boards and IC tape carriers, and suspensions for hard disk drives.
Flexible printed wiring boards, characterized by flexibility, can be accommodated in bent or twisted configurations in the housings of electronic equipment. In electronic appliances being made more and more compact, flexible printed wiring boards permit effective utilization of space. In addition, they can meet wiring needs for movable parts. In the field of integrated circuits, packages using tape carriers of copper foil have been employed that can be finished to finer pitches than ever to keep pace with the tendency toward thinner and shorter packages.
Flexible printed wiring boards and IC tape carriers are fabricated by laminating copper foil to a base of polyimide or other resin and applying heat and pressure, as well as adhesive to the laminate, to provide a unitary structure. The circuit boards are capable of containing etched wiring patterns as fine in pitch as 50 to 100 .mu.m.
Further, for wiring on a suspension supporting magnetic heads of a hard disc, the instances where copper alloy foils are used instead of conventional lead wires are increasing. In such applications, higher strength copper alloy foils are required, since in head assembly, it is necessary that the foils not deform during handling. The demands of higher speed signal transfer requires higher electric conductivity.
Copper is a metal with excellent electric conductivity, and in the aforementioned field where high conductivity is a prime consideration, copper of about 99.99% purity is commonly used. However, copper has the disadvantage of markedly lowering strength and heat resistance with increased purity, and the heat applied in lamination to a flexible resin substrate of polyimide or the like can cause deformation or breakage of pure copper foil. This adversely affects the reliability of the products. Apart from considerations of thermal history during fabrication, copper foil requires heat and heat shock resistance when used in hot environments, e.g., in the engine compartment of an automobile, where the temperature is around 100.degree. C.
On the other hand, when the thickness of copper foil in manufacturing is decreased to that of the laminate foil, difficulties in foil rolling with good yield arise. Internal defects such as inclusions, in particular, can cause rupture or pinhole generation on rolling, leading to reduced productivity and hence increased manufacturing cost. For these reasons, a copper stock with as small inclusion contents as possible is called for.
In recent years, copper alloys of the precipitation type have found increasing use in applications that demand high strength and electric conductivity, as in copper alloys for electronic equipment. Cu--Cr--Zr alloys are typical of the precipitation copper alloys that combine high strength with high conductivity, and efforts have been made to put them into practical use as materials for electronic equipment. According to research thus far made on these alloys, aging precipitation produces fine Cr and Cu.sub.3 Zr particles precipitated in a copper matrix. This helps bring about increased strength and conductivity, and the size of precipitated particles which contributes to the strength increase is not more than 0.5 .mu.m.
For example, Japanese Patent No. 2501275 (registered Mar. 13, 1996) discloses a copper alloy having both conductivity and strength. The alloy is characterized by a composition including 0.01-2 wt % chromium and/or 0.005-1 wt % zirconium, not more than 60 ppm oxygen, the balance being substantially copper, wherein precipitates not greater than 50 .mu.m in size are present at the rate of 100-100000 pieces per square millimeter. The patent also teaches that specified amounts of Ni, Sn, Fe, Co, Zn, Ti or/and many other alloying elements may be added. However, a material containing such large precipitates on the order of 50 .mu.m in size, numbering as many as 100-100000 pieces/mm.sup.2, is not utilizable in making a rolled foil.
The phenomenon observed with the Cu--Cr--Zr copper alloys that fine Cr and Cu.sub.3 Zr precipitated particles formed in the copper matrix increase the strength and conductivity is attributable to the fact that the additional elements Cr and Zr by nature can scarcely form solid solutions with copper. On the other hand, coarse crystallization or precipitation products that do not add to strength tend to remain in the matrix. Moreover, these additional elements have such high activities that they easily form oxides, sulfides, silicides, etc., with the result that a structure is easily formed in which these rather large particles are dispersed in the matrix. In fact, the above Patent No.2501275 describes that precipitates 0.5-50 .mu.m in size were present at the high rate of 100-100000 pieces/mm.sup.2.
Cu--Cr--Zr copper alloys are basically excellent copper alloys having both high strength and great electric conductivity. They are promising materials for rolled copper foils. However, an obstacle on the way to the practical use of the Cu--Cr--Zr copper alloys as copper alloy foil materials is the possibile presence of the coarse particles. The coarse particles can cause ruptures and pinholes at the time of rolling, resulting in low productivity and high manufacturing cost.
There has been a demand, therefore, for the development of Cu--Cr--Zr copper alloy foils having excellent strength, conductivity, heat resistance, and productivity suited for use in fabricating flexible printed boards, IC tape carriers and the like. Also, it is necessary to prevent separation of such copper alloy foils from bases, such as flexible printed boards, at solder-bonded portions by the heat generated during operation.